Hundreds of Native American protectors have gathered at the site since April to protest the pipeline’s construction on land they claim is tribal under the 1851 Treaty of Fort Laramie.

“I urge you to take all appropriate measures to protect the safety of the Native Americans protesters and their supporters who have gathered peacefully to oppose the construction of the pipeline,” Sanders wrote in the letter.

Sanders asks that President Obama direct the Justice Department to send observers to the site to protect protestors’ safety and First Amendment rights; call North Dakota Governor Jack Dalrymple to remove the National Guard from the protest camp; and direct the Army Corps of Engineers to issue an order to stop work on construction of the pipeline near the protest site to reduce tensions while awaiting judicial action.

Sanders again called on the president to suspend construction of the pipeline until the Army Corps of Engineers completes a full cultural and environmental review.

“It is deeply distressing to me that the federal government is putting the profits of the oil industry ahead of the treaty and sovereign rights of Native American communities,” Sanders wrote. “Mr. President, you took a bold and principled stand against the Keystone pipeline – I ask you to take a similar stand against the Dakota Access Pipeline.”

Sanders previously called on the president to block construction of the pipeline during a rally outside the White House with leaders of the Standing Rock Sioux tribe and other tribal nations in September.